More like half a dozen people and something like 12-18 months (jgraham will correct me) to make the world's fastest JS JIT. But your bigger point stands.
Carakan was 16.5 months from first commit to shipping (and a month or so more to not being notably buggy), with a team of five for the vast majority of that time. The only code carried over verbatim from Futhark was the regexp engine (though that had machine-code generation added to it), and the parser was also pre-existing (though not shared with Futhark!).
For comparison: MS were the last to do a major rewrite of their layout engine, which started, AIUI, before IE7 shipped (Oct 2006) and shipped in IE8 (Mar 2009), and more-or-less reached feature parity with others with IE9 (Mar 2011). To be fair, they were working from a far more archaic codebase than anyone else, so probably had more work, but they also had a far larger team than anyone else working on this.
When discussing timeframes, it's important to keep the moving goalposts in mind.
Back in 2009 it took order of 16 months to write a competitive JIT. But today's JITs are a good bit faster than they were in 2009, which means more special cases that need to be considered and made fast, from what I've seen of JIT development.
I should also note that I included QA resources of various sorts in my team size estimate....
Right: I think if Carakan were done now it would likely take two years to being competitive, given the same team size (and I included QA in team size — varying between 1.5 and 2).
